TONY BLAIR is finished — no ifs, buts or maybes. The Prime Minister no longer has any chance of recapturing his popularity, regaining his credibility or restoring his authority. The only question is whether Mr Blair will use his speech at the Labour conference as a last chance to announce a dignified departure date, around next May’s local elections, or whether he will have to be dragged kicking and screaming out of Downing Street before Christmas à la Margaret Thatcher, with men in grey suits doing the dirty work, and men in white coats standing a figurative step behind them. As in many other respects, Mr Blair seems eager to follow Mrs Thatcher’s example.

How can I assert so unequivocally that Mr Blair is finished? After all, Gordon Brown is quiescent and his chief lieutenants are urging other Brownites not to cause political trouble.

Meanwhile, the Prime Minister’s supporters declare that he is brimming with energy, is planning all sorts of eye-catching policy initiatives and is absolutely determined not to “become a lame duck”. How, then, can I be so sure that Mr Blair will either announce his departure date in the month ahead or will be forcibly removed by Christmas? For the same reason that a decent chess player can look at a seemingly complex endgame and say without hesitation: “Black checkmates in three moves.”

Statements about British politics that were still speculative or controversial before the summer have now acquired the ineluctability of mathematical logic. Endgames are like that. The clutter of irrelevant pieces has been cleared off the chessboard and the game is reduced to its essentials.

Three events since the summer have had this decluttering effect: the war in Lebanon, the terrorist scare at British airports and the “open microphone” debacle in St Petersburg, when Mr Blair went beyond the wildest parody in his sycophantic fawning to President Bush. These three disasters destroyed what little hope Mr Blair might have had of restoring a modicum of support for his post- Iraq foreign policies or regaining a modicum of respect from Labour voters who feel shamed, disgusted and betrayed by the Blair-Bush relationship.

The combination of these events has transformed the political chess game, both for Mr Blair and for his party. Starting with the outright anti-Blairites on the Left, they no longer have any reason to tolerate Mr Blair since he appears to be leading the party to electoral disaster, while the arguments for an immediate putsch are becoming overwhelming — especially if you hold him personally responsible for the deaths of thousands of innocent people and fear that he may be plotting some even greater outrage with President Bush. For the Left, therefore, ousting Mr Blair is no longer just a political priority; it is a moral emergency.

That may be no surprise, but the big change over the summer has been in the way the game now looks to Mr Brown, along with the great bulk of moderate Labour ministers and voters. Until a few months ago they were willing to tolerate Mr Blair for another year, or maybe even two, as the price that had to be paid for party unity and an orderly transfer of power. But Mr Blair no longer offers any hope of stability. On the contrary, his foreign policy threatens to engulf the country and the Government in additional crises at any moment. With his credibility and Labour’s popularity eroding by the day, Mr Blair only offers an orderly transfer of weakness.

To make matters worse, Mr Blair no longer offers an orderly transition of any kind. His judgment is now so erratic, not only in foreign policy but even in practical issues such as last June’s Cabinet reshuffle, that he may not be willing to cede power gracefully at all.

Like Mrs Thatcher in her waning days, Mr Blair is widely believed to have “lost his marbles” — and the evidence against him is far more damning than the evidence for Mrs Thatcher’s “madness” ever was. The poll tax may have been unpopular and stupid, but it did not involve starting wars. And Mrs Thatcher’s occasional rudeness to European politicians was never a humiliation like Mr Blair’s subservience to President Bush.

So widespread is the loathing of Mr Blair that a putsch against him would no longer need to be spearheaded by the Labour Left. This means that Mr Brown is in a position to accelerate the Prime Minister’s departure without making any promises to Labour activists and unions that he might later regret. In fact, by distancing himself from President Bush’s foreign policy as soon as he become Prime Minister, Mr Brown could immediately help to restore the Government’s popularity and simultaneously win himself all the gratitude he needs from the Labour Left.

The incentives for Mr Brown and other Labour politicians for an early transfer of power are therefore overwhelming. But what about Mr Blair? Surely the Labour Party rules make it almost impossible to oust a leader against his will? The answer is that no Prime Minister can remain in office once he has visibly lost the support of his Cabinet — and a Cabinet rebellion like the one that toppled Mrs Thatcher looks likely if Mr Blair refuses to name the date of his departure.

Under these circumstances, the best policy for Mr Blair would be to announce now a firm date for his retirement, most likely March or April next year. If he did this, he could avert any rebellion, since nobody would take the trouble to oppose him merely to accelerate his departure by a month or two. If he did this he could enjoy a dignified last few months in office and would become no more a lame duck than the many American and French presidents who have remained effective politicians right up to the end of their fixed terms. For Mr Blair, announcing a departure date is his only rational option. If he refuses to name a date by the Labour conference, this in itself will be the clearest possible sign of his erratic judgment; if you like, his Thatcher-like “madness” — and the best possible reason for a Thatcher-style Cabinet coup.

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